Conventionally, an image is formed on a sheet (paper) using an image forming apparatus such as a MFP (Multi-Function Peripheral). Further, in order to erase an image formed on a sheet to reuse the sheet, an image is printed on the sheet with a color-erasable coloring agent such as ink containing leuco dye.
The color-erasable coloring agent can be erased by heating at a sufficiently high temperature. Thus, to be reused, the sheet is heated using an erasing apparatus to erase the image formed thereon. The aforementioned erasing of an image formed on a sheet is hereinafter referred to as ‘color erasing’.
In the erasing apparatus, a heat roller and a press roller are oppositely arranged across a sheet conveyance path, and a sheet is conveyed and heated between the heat roller and the press roller, thereby erasing a color-erasable coloring agent. Further, two erasing units, each consisting of the heat roller and the press roller, are respectively arranged at the upstream side and the downstream side of the sheet conveyance path to erase colors on both sides of the sheet.
However, if the erasing temperature of the erasing unit at the downstream side is increased to completely erase colors of the images on both sides of a sheet through a single color erasing operation, a problem arises in that the sheet is curled, which leads to a paper jam in the erasing apparatus.
It is deemed that the reason for the curl of the sheet lies in the change in the stretching rate of the sheet caused by the difference in the moisture between the front side and the back side of the sheet. The more the erasing temperature of the erasing unit at the downstream side is increased, the elasticity difference between the front surface and the back surface of the sheet increases, and the curl amount increases.